Blocky Textures around object, Blender render artifacts, possible causes?
The problem was: assigning a UV Map to the Cylinder was somehow messing up the floor's texture or UV Map. I don't know why. But I was able to clear the artifacts from the render:

I fixed it by selecting the floor --> Edit Mode, and applying a UV Map to it, via the left panel (Shading / UVs tab) --> UVs section --> Unwrap. So both the Cylinder and floor have their own UV Maps.



I don't know why changing the cylinder created artifacts on the floor; they are two different objects, two different UV Maps, two different materials, two different textures. But this solution worked for me!